Frame color
The frame color is the color that shows up in the parts of the Flash viewer when you are a in a room whose backdrop does not fill up the entire screen.
You can click on the Room icon 
in the control bar and select "Frame color..." to change your preference.
The background color of a room will be used for the frame surrounding the backdrop if you have selected "Use place color".
You can also select "Use custom" and then click on the square to pick your own custom color for the frame. You can either select a background color from the swatches that show up when you click on the background color square or you can type a hexadecimal code for a specific color into the white text box.
Hexadecimal Codes
Many graphics editors will tell you the hexadecimal code for the color you are using. You can also find the codes for various colors on some websites such as this color mixer or from a chart such as this one.
See Also
- Edit your room - for information on changing the background color in your rooms